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How the birth of Earth made a planet fit for life

15 Feb 2017 - 10:00

Our solar system was formed from a minor swirling offshoot during a supernova explosion, such as the Carina Nebula.

Our little solar neighbourhood, situated two-thirds of the way out on a spiral arm of the Milky Way, came to be 9.2 billion years after the big bang, or 4.6 billion years ago. The sun and all the planets that make up our solar system most likely originated as a relatively minor swirling offshoot of dust and gas from an exploding supernova. Most of the dust and gas of this spiralling offshoot was pulled by gravity toward the centre of the spiral. The amount of heat released from the gravitational collapse was enough to ignite the sun, which is composed mostly of hydrogen and helium in proportions similar to the rest of the universe. Gases and dust in the outer, colder regions of the swirling disc rapidly coalesced into the ‘dirty snowball’ outer planets: Jupiter, Saturn, Uranus and Neptune.

Mercury, Venus, Earth, Mars and the asteroid belt form the inner rocky planets; Jupiter, Saturn, Uranus and Neptune form the ‘dirty snowball’ outer planets (distances not to scale).

Relatively dense residual bits of metal and rock were left between the sun and the outer snowball planets. This metal and rock, which started out as supernova dust, condensed into small rocks that collided into yet larger bodies that in turn attracted other bits until they had amassed into planetesimals (small planets). Planetesimals eventually coalesced into a few dozen planetary bodies ranging from a hundredth to a tenth as massive as Earth. More gradually these planetary bodies combined through a series of multiple giant impacts and formed the small rocky inner planets: Mercury, Venus, Earth and Mars. The condensation of the swirling rings of dust into the rocky planets is estimated to have taken place somewhere between 63 and 127 million years after the birth of our solar system 4 567 million years ago, such that Earth was fully formed sometime between 4 504 and 4 440 million years ago.

A colossal collision

The final, planet-defining impact in Earth’s formation was with a planetary body the size of Mars. This colossal collision had several long-lasting consequences: it tilted Earth’s spinning axis by 23.5 degrees from the vertical, which give us seasons; it sent rock debris into orbit, where it aggregated to give us our moon; and it captured most of the impactor’s iron core to give Earth an unusually large amount of iron and an enduring magnetic field.

Hence, Earth represents a small, highly distilled concentrate of all the elements heavier than hydrogen and helium, which makes it a rarefied collective condensate of elements that otherwise make up on average only 2% of our entire solar system. Elements like iron, silicon, oxygen and magnesium are the most abundant, but all the stable elements that can exist are to be found here on Earth. Most importantly for life, these elements, which were hastily thrown together by collisions, were soon further distilled. The lightest elements percolated to the surface. Hydrogen and helium gas were too light to be held by Earth’s gravity and escaped into space. But water, nitrogen and carbon dioxide accumulated in the atmosphere and oceans, which, along with rocks exposed at the surface, held an abundance of all the ingredients necessary for life.

While the lightest elements rose to the surface, the heaviest among them, mostly dense molten blobs of iron and nickel, sank relatively quickly to the centre due to gravity. Earth has since cooled down and has crystallised out a solid iron-nickel inner core. However, much of the iron-rich liquid remains as the outer core and envelops the solid inner core. The continuous roiling about of the outer core’s liquid iron acts like a dynamo that generates a magnetic field. This magnetic field is critical to making Earth habitable because it shields the planet from high-energy solar winds that are harmful to life. Therefore, the early rapid distillation of the bulk Earth that included a dense iron-rich core capable of generating a magnetic field and an outermost envelope of light gas and water, made it a place conducive to life.

Initially Earth was too hot for life, forming a semi-liquid magma ball due to the huge amount of energy released as heat from the impacts that formed it, as well as heat released by the decay of unstable, short-lived radioactive elements left over from the supernova. But within 100 million years after final formation, Earth’s surface had cooled down enough for it to crust over and for much of the water vapour to rain out as oceans. It would appear, then, that Earth was habitable as early as 4.4 billion years ago. However, if life did exist, it would have had to survive major cataclysmic events during the Hadean – the first geological eon (named after Hades, the ancient Greek god of the underworld).

The hellish Hadean eon

Hadean Earth.

It was throughout the Hadean, between 4.5 and 4 billion years ago, that conditions on Earth were particularly hellish, owing in large part to the final sweeping up of the substantial rock bodies lingering in our planet’s orbit. Episodic collisions continued to pummel Earth with sufficient energy to melt and recycle much of the thin solid crust as well as repeatedly boil off much of the oceans. Even now, Earth continues to receive as much as 100 metric tons of debris from space daily, most arriving as innocuous shooting stars. On occasion, though, a bigger, more threatening impactor arrives – like the one 10 km in diameter that wiped out the dinosaurs. But by 4 billion years ago, the late heavy-bombardment period was largely over and Earth presented a far more conducive place for life to take hold. And yet, even after the Hadean bombardments had ceased, Earth was not a completely placid place for life.

Earth had back then, as it does today, its own internal rumblings, which were expressed at the surface in the form of earthquakes and volcanic eruptions. Earthquakes and volcanoes result from movement deep within Earth. In the same way as your cup of hot coffee cools, the most efficient way for a hot Earth to cool is by the hottest, deepest and least dense material rising towards the surface while the coolest, most dense material that is closest to the surface sinks into the hot interior. This slow churning (convective overturning) of Earth occurs in its liquid iron outer core as well as in the thick overlying mantle rocks, where solid rocks slowly flow like hot wax. The mantle is capped by a skin-of-an-apple-thin crust that is divided up into 15 large individual rigid plates. Plate tectonics is the continuous movement of these crustal plates relative to one another, movement that is ultimately driven by the convective overturning of deep Earth.

In plate tectonics, plumes of hot, buoyant mantle rock rise up to generate magmas that form new oceanic crust at the surface. New oceanic crust forms along the mid-ocean ridge, a winding submarine volcanic mountain chain that runs like the seam of a baseball for 65 000 kilometres along the sea floor throughout the world’s oceans. As new magma comes up, it forces the existing oceanic crust to move aside and the ocean basins widen. Eventually the oceanic crust cools and becomes dense enough to sink back down into the mantle and ocean basins shrink. This overturning of mantle rock may have initially been, as in your cup of coffee or a lava lamp, dominated by vertical up and down movement. It was only once Earth had cooled enough, by around 3.3 to 3 billion years ago, that rigid crustal plates with large continents had formed and resulted in less vertical and more sideways crustal movement such as we observe today.

The continents that ride high above the ocean basins represent the accumulation of the lightest rocks, those too buoyant to be recycled back into the mantle. They are forever floating like scum on the surface, episodically colliding together or breaking apart. Continents started out small, but over time they grew from the accumulation of lighter elements as they percolated to the surface. But the barren, high-and-dry rocky surfaces of continents would only much later be home to a highly diverse assemblage of life forms.

Continual recycling of the crust by plate tectonics means that the oldest rocks are the least likely to survive and, as a result, most of early Earth history has been erased. The oldest bit of Earth’s crust dated thus far is a 4.4-billion-year-old zircon, a mineral commonly found in continental rocks and known for its ability to survive rock recycling. The oldest rock, a metamorphic rock called gneiss, is just over 4 billion years old, and the oldest oceanic sedimentary rocks known were deposited 3.85 billion years ago. These ages suggest that Earth had become a place suitable for life at least several hundred million years before the first indirect evidence of life is found in rocks 4.1 to 3.8 billion years old, and before the first direct fossil evidence of life is found in rocks 3.7 billion years old.

John S. Compton is an associate professor in the Department of Geological Sciences. This is an extract from his latest book Human Origins, published by Earthspun.

The launch of Human Origins will take place Thursday 2 March, 18h00 for 18h30 at the University of Cape Town’s upper campus, John Day Building, Lecture Theatre 2. All are invited to attend at no charge, but please RSVP to John Compton for catering purposes and any queries.